The Structure of Evolutionary Theory Read online

  Although the next few paragraphs will be the most vague and impressionis­tic (I trust) of the entire book, I venture these ill-formulated statements about Zeitgeist because I feel that something important lurks behind my inability to express these inchoate thoughts with precision. I argue above (page 14) that the key concerns of the three essential branches of Darwinian logic might be identified as agency, efficacy and scope of natural selection. In each of these domains, I believe, the revised structure of evolutionary theory, as presented in this book, might be characterized as expansion and revision according to a set of coordinated principles, all consonant with our altered Zeitgeist vs. the scientific spirit of Darwin's own time. The modern revision seeks to replace Darwin's unifocal theory of organismic selection with a hierarchical account (leg one); his unidirectional theory of adaptational construction in the func­tionalist mode with a more balanced interaction of these external causes, treating internal (or structural) constraints primarily as positive channels, and not merely as limitations (leg two); and his unilevel theory of micro-evolutionary extrapolation with a model of distinctive but interacting modes of change, each characteristic for its tier of time. In short, a hierarchy of inter­acting levels, each important in a distinctive way, for Darwin's single locus; an interaction of environmental outsides with organic insides for Darwin's single direction of causal flow; and a set of distinctive temporal tiers for Dar­win's attempt to situate all causality in the single microevolutionary world of our own palpable moments.

  I do sense a common underlying vision behind all these proposed reforms. Strict Darwinism, although triumphant within mid 20th century evolutionary theory, embodied several broad commitments (philosophical or metatheoretical, in the technical sense of these terms), more characteristic of 19th than of 20th century thought (and, obviously, not necessarily wrong, or even to be discounted, for this reason — as nothing can be more dangerous to the prog­ress of science than winds of fashion, and we do, after all, learn some things, and develop some fruitful approaches, with validity and staying power well beyond their time of origin and initial popularity). Some aspects of Darwin's formulation broke philosophical ground in a sense quite consonant with our modern Zeitgeist of emphasis upon complexity and interaction — particularly, Darwin's focus on the interplay of chance and necessity in sources of varia­tion vs. mode of selection. Indeed, Darwin paid the usual price for such inno­vation in the failure of nearly all his colleagues, even the most intellectually acute, to grasp such a radical underlying philosophy. But, in many command­ing respects, Darwinism follows the norms of favored scientific reasoning in his time.

  The logic of Darwin's formulation rests upon several preferences in scien­tific reasoning more characteristic of his time than of ours — preferences that [Page 31] many scientists would now view as unduly restrictive in their designation of a privileged locus of causality, a single direction of causal flow, and a smooth continuity in resulting effects. Classical Darwinism follows standard reduc­tionist preferences in designating the lowest level then available — the organ­ism, for Darwin — as an effectively unique locus of causality (the first leg of agency). In this sense, the efforts of Williams and Dawkins (see Chapter 8) to reduce the privileged locus even further to the genic level (perforce unavail­able to Darwin) should be read as a furthering and intensification of Darwin's intent — in other words, a basically conservative adumbration of Darwin's own spirit and arguments, and not the radical conceptual revision that some have imagined.

  At this single level of causality, classical Darwinism then envisages a similarly privileged direction of causal flow, as information from the environment (broadly construed, of course, to include other organisms as well as physical surroundings) must impact the causal agent (organisms struggling for repro­ductive success) and be translated, by natural selection, into evolutionary change. The organism supplies raw material in the form of “random” varia­tion, but does not “push back” to direct the flow of its own alteration from inside. Darwinism, in this sense, is a functionalist theory, leading to local ad­aptation as the environment proposes and natural selection disposes. Finally, classical Darwinism completes a trio of privileged causal places and conse­quently directional flows by postulating strict continuity in results, as local selection scales smoothly through the immensity of geological time to engen­der life's history by pure extrapolation of lowest-level modes and causes.

  By contrast, the common themes behind the reformulations defended in this book all follow from serious engagement with complexity, interaction, multiple levels of causation, multidirectional flows of influence, and pluralis­tic approaches to explanation in general — a set of integrated approaches that strongly contribute to the Zeitgeist of our moment. To anticipate and make a preemptive strike against the obvious counterattack from Darwinian tradi­tionalists, these alternative themes do not substitute a “laid back, laissez-faire, anything goes” kind of sloppy tolerance for contradiction and fuzziness in argument against the genuine rigor of old-line Darwinism. The social and psychological contributions of a Zeitgeist to the origin of hypotheses bear no logical relationship to any subsequent scientific defense and validation of the same hypotheses. Moreover, on this subject of test and confirmation, I es­pouse a rigorously conventional and rather old-fashioned “realist” view that an objective factual world exists “out there,” and that science can access its ways and modes. Whatever the contribution of a Victorian Zeitgeist to Darwin's thinking, or of a contemporary Zeitgeist to our revisions, the dif­ferences are testable and subject to validation or disproof by the usual arma­mentarium of scientific methods. That is, either Darwin is right and effec­tively all natural selection occurs at the organismic level (despite the logical conceivability of other levels), or the hierarchical theory is right and several levels make interestingly different and vitally important simultaneous [Page 32] contributions to the overall pattern of evolution. The same ordinary form of testability can be applied to any other contrast between strict Darwinism and the revised and expanded formulations defended in this book.

  As the most striking general contrast that might be illuminated by reference to the different Zeitgeists of Darwin's time and our own, modern revisions for each essential postulate of Darwinian logic substitute mechanics based on in­teraction for Darwin's single locus of causality and directional flow of effects. Thus, for Darwin's near exclusivity of organismic selection, we now propose a hierarchical theory with selection acting simultaneously on a rising set of levels, each characterized by distinctive, but equally well-defined, Darwinian individuals within a genealogical hierarchy of gene, cell-lineage, organism, deme, species, and clade. The results of evolution then emerge from complex, but eminently knowable, interactions among these potent levels, and do not simply flow out and up from a unique causal locus of organismal selection.

  A similar substitution of interaction for directional flow then pervades the second branch of selection's efficacy, as Darwin's functionalist formulation — with unidirectional flow from an external environment to an isotropic or­ganic substrate that supplies “random” raw material but imposes no direc­tional vector of its own to “push back” from internal sources of constraint — yields to a truly interactive theory of balance between the functionalist Dar­winian “outside” of natural selection generated by environmental pressures, and a formalist “inside” of strong, interesting and positive constraints gen­erated by specific past histories and timeless structural principles. Finally, on the third and last branch of selection's range, the single and control­ling microevolutionary locus of Darwinian causality yields to a multileveled model of tiers of time, with a unified set of processes working in distinctive and characteristic ways at each scale, from allelic substitution in observable years to catastrophic decimation of global biotas. Thus, and in summary, for the unifocal and noninteractive Darwinian models of exclusively organismal selection, causal flow from an environmental outside to an organismal in­side, and a microe
volutionary locus for mechanisms of change that smoothly extrapolate to all scales, we substitute a hierarchical selectionist theory of numerous interacting levels, a balanced and bidirectional flow of causal­ity between external selection and internal constraint (interaction of func­tionalist and structuralist perspectives), and causal interaction among tiers of time.

  Among the many consequences of these interactionist reformulations, punctuational rather than continuationist models of change (with stronger structuralist components inevitably buttressing the punctuational versions) may emerge as the most prominent and most interesting. The Darwinian me­chanics of functionalism yield an expectation of continuously improving lo­cal adaptation, with longterm stability representing the achievement of an optimum. But interactionist and multi-leveled models of causality reconcep­tualize stasis as a balance, actively maintained among potentially competing forces at numerous levels, with change then regarded as exceptional rather than intrinsically ticking most of the time, and punctuational rather than [Page 33] smoothly continuous when it does occur (representing the relatively quick transition that often accompanies a rebalancing of forces).

  To end this admittedly vague section with the punch of paradox (and even with a soundbite), I would simply note the almost delicious irony that the for­mulation of a hierarchical theory of selection — the central concept of this book, and invoking a non-vernacular meaning of hierarchy in the purely structural sense of rising levels of inclusivity — engenders, as its most impor­tant consequence, the destruction of a different and more familiar meaning of hierarchy: that is, the hierarchy of relative value and importance embod­ied in Darwin's privileging of organismic selection as the ultimate source of evolutionary change at all scales. Thus, a structural and descriptive hierarchy of equally effective causal levels undermines a more conventional hierarchy of relative importance rooted in Darwin's exclusive emphasis on the micro-evolutionary mechanics of organismal selection. And so, this structuralist view of nature's order enriches the structure of evolutionary theory — carry­ing the difference between strict Darwinism and our current understanding through more than enough metatheoretical space to fashion a Falconerian, not merely a Darwinian, rebuilding and extension for our edifice of coherent explanation.

  A PERSONAL ODYSSEY

  For reasons beyond mere self-indulgence or egotism, I believe that defenders of such general theories about large realms of nature owe their readers some explanation for the personal bases and ontogeny of their choices — for at this level of abstraction, no theory can claim derivation by simple logical or em­pirical necessity from observed results, and all commitments, however well defended among alternative possibilities, will also be influenced by authorial preferences of a more contingent nature that must then be narrated in order to be understood. Moreover, and in this particular case, the structure of this book includes a set of vigorously idiosyncratic features that, if not acknowl­edged and justified, might obscure the far more important raison d'etre for its composition: the presentation of a tight brief for substantial reformulation in the structure of evolutionary theory, with all threads of revision conceptually united into an argument of different thrust and form, but still sufficiently con­tinuous with its original Darwinian base to remain within the same intellec­tual lineage and logic.

  Two aspects of my idiosyncratic procedures require explicit commentary here because, at least as my intention, they should reinforce this book's cen­tral argument for coherence (logical, historical and empirical) of the revised and general structure of evolutionary theory, and not further the opposite, al­beit customary, function of such “confessional” writing — namely, to slake authorial egos, fight old battles, and relate twice-told tales to one's own ad­vantage (although I claim no immunity from these all too human foibles).

  This book will be published in the Spring of 2002, an auspicious and palindromic year just one step out of the starting gate for a new millennium. [Page 34] At the same time, and fortuitously, my 10th and last volume of monthly es­says in Natural History Magazine, written without a single break from Janu­ary 1974 to January 2001, will also appear in print. In an eerie coincidence (with no meaning that I can discern), my first technical book, Ontogeny and Phylogeny, appeared exactly 25 years before, in 1977, also at the same time as my first book of Natural History essays, Ever Since Darwin. This odd and twofold simultaneous appearance, 25 years apart, of my best youthful efforts in the contrasting (but not really conceptually different) realms of technical and popular science, and then of my best shots from years of greater maturity in the same two realms, has forced me to think long and hard about the meaning of continuity, commitment and personal perspective.

  My popular volumes fall into the explicit and well recognized category of essays, a literary genre defined, ever since Montaigne's initiating 16th century efforts, as the presentation of general material from an explicitly personal and opinionated point of view — although the best essays (literally meaning “attempts,” after all) tend to be forthright in their expression of opinions, generous (or at least fair) to other views, and honest in their effort to specify the basis of authorial preferences. On the other hand, technical treatises in science do not generally receive such a license for explicitly personal expres­sion. I believe that this convention in technical writing has been both harmful and more than a bit deceptive. Science, done perforce by ordinary human beings, expressing ordinary motives and foibles of the species, cannot be grasped as an enterprise without some acknowledgment of personal dimen­sions in preferences and decisions — for, although a final product may display logical coherence, other decisions, leading to other formulations of equally tight structure, could have been followed, and we do need to know why an author proceeded as he did if we wish to achieve our best understanding of his accomplishments, including the general worth of his conclusions.

  Logical coherence may remain formally separate from ontogenetic con­struction, or psychological origin, but a full understanding of form does re­quire some insight into intention and working procedure. Perhaps some pre­sentations of broad theories in the history of science — Newton's Principia comes immediately to mind — remain virtually free of personal statement (sometimes making them, as in this case, virtually unreadable thereby). But most comprehensive works, in all fields of science, from Galileo's Dialogo to Darwin's Origin, gain stylistic strength and logical power by their suffu­sion with honorable statements about authorial intents, purposes, prejudices, and preferences. I cannot think of a single major book in natural history — from Buffon's Histoire naturelle and Cuvier's Ossemens fossiles to Simpson's Tempo and Mode, and Mayr's Animal Species — that does not include such extensive personal information, either in explicit sections, or inserted by-the-by throughout. (Even so abstract a presentation as R. A. Fisher's Genetical Theory of Natural Selection gains greatly in comprehension through its long and final, if in retrospect regrettable, section on the author's idiosyncratic eugenical views about human improvement.) I have included personal discus­sion throughout this text, but let me also devote a few explicit pages to the [Page 35] two points that I regard as most crucial to understanding the general argu­ment through (or despite) conscious idiosyncrasies in my presentation.

  History

  Many technical treatises in science begin with a short section on previous his­tory of work in the field — usually written in the hagiographical mode to de­pict prior history as a march towards final truths revealed in the current vol­ume. Sometimes, authors get a bit carried away, and these historical sections expand into substantial parts of the final book. Lest anyone make the false in­ference that my full first half of history arose in this haphazard and initially unintended way, I hasten to assure readers that my final result was my inten­tion from the start.

  For several reasons, I always conceived this book as a smooth joining of two halves, roughly equal in length and importance. First, and ontogenetically, I had written my earlier technical book, Ontogeny an
d Phylogeny, in this admittedly unusual manner — and I remain pleased with both the distinctiveness and the efficacy of the result. Second, I believe that the history of evo­lutionary thought, and probably of any other subject imbued with such im­portance to our lives and to our understanding of nature, constitutes an epic tale of fascinating, and mostly honorable, people engaged in a great struggle to comprehend something very deep and very difficult. Thus, such histories capture a bit of the best in us (also of the worst, but all human endeavors so conspire) — a bit, moreover, that cannot be expressed in any other way. We re­ally do need to honor the temporal substrate of our current understanding, not only as a guide to our continuing efforts, but also as a moral obligation to our forebears.

  But a third and practical reason trumps all others. Although I would not state such a claim as a generality for all scientific analyses, in this particular case I do not see how the structure of evolutionary theory can be resolved and the appropriate weights of relative importance assigned to the different com­ponents thereof, absent such a historical perspective. (Would it not be odd to claim, in any case, that the quintessential science for resolving the nature of life's history can itself be understood as a pristine construction, a fully-formed conceptual entity drawn intact from some analog of Zeus's brow, rather than an “organic” structure of ideas with its own ontogeny and history?)

  To give one example at the largest and at the smallest scales of my argu­ment, I don't know how I could have properly defended my identification and explication of the threefold essence of Darwinian logic without documenting the history of theoretical debate in order to tease out the components that have always been most troubling, most central, and most directive. A pure description of the theory's abstract logic simply will not suffice. To epitomize, I have identified these essential components on three basic grounds: that logic compels (Chapter 2), that history validates (Chapters 3–7), and that current debate reaffirms (Chapters 8–12). The middle term of this epitome unites the end members; I cannot present a coherent or compelling defense without this linkage. The three issues of agency, efficacy and scope build the Darwinian [Page 36] essence both because the logical structure of the theory so dictates, and because the history and current utility of the theory so document.